In an attempt to increase the efficiencies and performance of contemporary jet engines, and gas turbine engines generally, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure, in a corrosive environment, that are now frequently specified place increased demands on engine components and materials. Indeed the gradual change in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine.
The turbine blade is one engine component that directly experiences severe engine conditions. Turbine blades are thus designed and manufactured to perform under repeated cycles of high stress and high temperature. An economic consequence of such a design criteria is that currently used turbine blades can be quite expensive. It is thus highly desirable to maintain turbine blades in service for as long as possible. It is correspondingly desirable to manufacture and finish turbine blades so as to withstand the corrosive and erosive forces that will attack turbine blade materials.
Turbine blades used in modern jet engines are frequently castings from a class of materials known as superalloys. The superalloys include alloys with high levels of cobalt and/or nickel. In the cast form, turbine blades made from superalloys include many desirable mechanical properties such as high strength at elevated temperature. Advantageously, the strength displayed by this material remains present even under demanding conditions, such as high temperature and high pressure. Disadvantageously, with the optimization of mechanical properties, the superalloys generally can be subject to corrosion and oxidation at the high temperature operating regime. Sulfidation can also occur in those turbine blades subject to hot exhaust gases.
Thus, it has become known to provide coatings or protective layers on items, such as turbine blades, that are subject to corrosion, erosion or sulfidation. Chromium, aluminum, and other metallic diffused coatings can be used to provide a protective layer that is more resistant to corrosion and/or oxidation than is the underlying substrate material. In the case of superalloys, materials such as platinum, aluminum, and chromium can be used to provide a protective diffusion coating.
One method used for providing diffusion coatings is the pack cementation process. In this method the target, the industrial item to be coated, is placed in a box or retort with a “pack” surrounding it. The pack typically includes a source of the metal that is to be diffused into the target, inert packing material, and an activator if any. Typically the target lies in a bed of mixed powdered materials. The box containing the target and its surrounding pack is then placed in an oven where the materials are heated for a desired time at a desired temperature. Diffusion of desired elements takes place during the thermal cycle. Pack cementation is a comparatively attractive method of coating in that it is a relatively simple method that is relatively inexpensive to apply to the target, as compared to other methods of coating superalloys.
In the pack cementation process, elemental diffusion coatings on an article is produced through essentially a chemical vapor deposition procedure. The metallic elements in the pack react with the halide activator to form halide precursors which upon transport to the articles (substrates) react with the substrate surface to form the protective coatings. The material transfer reactions at the surface involve adsorption, dissociation and the various reactions involved in coating processes can become somewhat complex. Hence, several commercially practiced coatings involving more than one elemental diffusion utilize multiple sequential steps to diffuse single elements such as Cr, Al, and Si in order to achieve duplex coatings. The situation becomes increasingly more intricate with the need to diffuse more than two elements for coating formation in a single step.
The prior art methods of providing protective coatings have experienced limitations and drawbacks. One problem that has been encountered is the inability of known diffusion methods to apply a coating that includes as well other active elements (such as Hf, Si and Y) in addition to chromium. An improved oxidation, corrosion, and sulfidation resistance can be achieved in those coatings that include silicon, hafnium, yttrium, and other materials.
Hence there is a need for a method to apply an improved active element modified chromium diffusion protective coating on a metallic item such as a turbine blade. There is a need for an improved coating method that uses combinations of materials such as chromium, silicon, hafnium, and yttrium to render increased resistance to oxidation, corrosion, and sulfidation. Moreover there is a need for an improved diffusion method that retains the cost advantages associated with known pack cementation methods. The present invention addresses one or more of these needs.